An optical fiber comprises, as shown in the upper part of FIG. 1, a core 1 through which light propagates and a cladding 2 which surround the core. A refractive index n.sub.1 of the core 1 is larger than a refractive index n.sub.2 of the cladding as shown in the lower part of FIG. 1 so as to transmit light through the core. As a specific refractive index difference .DELTA.n defined by the formula: EQU .DELTA.n=(n.sub.2 -n.sub.1)/n.sub.1
is increased, an incident angle of the cladding surface at which total reflection occurs increases. Thereby, for example, power loss due to bending of the optical fiber is minimized.
Augmentation of the specific refractive index difference .DELTA.n can be accomplished, for example, by increasing the refractive index n.sub.1 of the core 1 by the addition of a metal oxide such as GeO.sub.2, Al.sub.2 O.sub.3 and TiO.sub.2 to a core portion of the glass preform corresponding to the core 1, and by decreasing the refractive index n.sub.2 of the cladding 2 by the addition of fluorine to a cladding portion of the preform corresponding to the cladding 2.
In the first method by which the refractive index n.sub.1 of the core 1 is increased, due to the increase of the amount of the additive, following drawbacks may be caused:
1. Addition of the additive induces light scattering (i.e. Rayleigh scattering) in proportion to the amount of the additive, and the light scattering undesirably increases attenuation.
2. Addition of a large amount of the additive tends to cause the formation of bubbles or single crystals in the glass preform. For example, when GeO.sub.2 is used as an additive, bubbles due to gaseous GeO.sub.2 may be formed. When Al.sub.2 O.sub.3 is used as an additive, it may form a cluster of its single crystals. The bubbles or the single crystals are the cause of the light scattering resulting in attenuation. In addition, they may be the cause of a fiber break.
The second method by which the refractive index n.sub.2 of the cladding is decreased can effectively overcome the above drawbacks. Typically, this method comprises adding the additive to the core portion of the soot preform to increase its refractive index to achieve a predetermined refractive index difference between the core portion and the cladding portion and adding the fluorine to the cladding portion to decrease its refractive index by heating the soot proform at a high temperature in an atmosphere of the fluorine-containing compound to produce the glass preform having a larger specific refractive index difference. However, this method has some drawbacks as follows:
By simply heating the soot preform at a high temperature in the atmosphere of the fluorine-containing compound, the core portion is also fluorinated so that the specific refractive index difference cannot be increased. Since fluorine is an inherently highly active material, it is very difficult to control the temperature in a furnace or to adjust the concentration of the fluorine-containing compound and the treating time so as to add fluorine only to the cladding portion of the preform. In order to add fluorine only to the cladding portion of the preform having conventional distributions of the additive and of a bulk density, it is necessary to control the temperature of the furnace within about .+-.30.degree. C. Thus, it is extremely difficult to produce an optical fiber having the desired specific refractive index difference.